Influence of diel biogeochemical cycles on carbonate equilibrium in a karst river

Abstract

Variations in temperature, photosynthesis, and respiration force diel variations in pH and dissolved CO 2 concentrations of surface streams, possibly controlling carbonate equilibrium between river water and carbonate stream beds. Diel cycles of water chemistry and δ 13C of dissolved inorganic carbon (DIC) were measured to assess how biogeochemical processes affect dissolution and precipitation of calcite and thus channel development in Ichetucknee River, a large spring-fed river (discharge > 6 m 3/s) flowing over carbonate karst terrain in north central Florida (USA). Samples were collected at a 4-h sampling interval during two one-week periods and at a 1-h interval during a single 24-h period. Simultaneously, temperature, pH, dissolved oxygen (DO) and NO 3 − concentrations were measured using in situ sensors at 15-min or 1-h intervals. Ca 2+, DIC and NO 3 − concentrations decreased during the day and increased at night causing diel changes of in-stream specific conductivity. These changes were inversely related to diel changes in pH, PCO 2 and DO concentrations. This work shows that photosynthesis and respiration of subaquatic vegetation are the dominant processes influencing in-stream diel variation. During the day, a simultaneous increase of δ 13C DIC and decrease in DIC indicates that photosynthesis was the primary control on DIC concentrations. Calcite saturation indices ranged from 0 to 0.5, with the highest value in daylight as a result of CO 2 consumption causing carbonate precipitation. The water remained saturated with respect to calcite at night and δ 13C DIC values decreased, indicating that CO 2 production from ecosystem respiration was the dominant process affecting DIC concentrations but was insufficient to induce significant carbonate dissolution. At night outgassing maintained in-stream DIC concentrations lower than the supersaturated DIC springs source but a drop in δ 13C DIC indicates that ecosystem respiration had a dominant influence over outgassing. Although CO 2 outgassing occurs, it is shown to be a minor component of the DIC mass balance while carbonate precipitation represents 88% of DIC loss. These results indicate that in-stream biological processes influence carbonate mineral diagenesis in large clear-water rivers.